G8 
but, on the contrary, oxygen disappears, and carbonic acid gas 
is evolved. In the dark, therefore, vegetables deteriorate ra- 
ther than purify the air, producing the same effect as the respi- 
ration of animals.” An ingenious explanation, supported by 
experiments, of the cause of these opposite effects, has been 
offered by Professor Burnet, which appears satisfactory. He 
considers that the influence of vegetation on the atmosphere is 
owing not to one, but to two functions, that is to the 
110 Digestion* and Respiration of Plants. Respiration is 
believed to proceed at all times as in animals, without intermic- 
sion, and its uniform effect is the production of carbonic acid ; 
while the former takes place only under the influence of light, 
and gives rise to evolution of oxygen gas, and the abstraction 
of carbonic acid. A plant exposed to sunshine, purifies the air 
by absorbing carbonic acid from the atmosphere, as well as 
that emitted by its own respiration, and emits oxygen gas in re- 
turn. In the dark, digestion is at a stand, and respiration con- 
tinuing without intermission, carbonic acid accumulates. From 
several of the preceding facts, it is supposed that the oxygen 
emitted by plants, while under the influence of light, is derived 
from the carbonic acid which they absorb, and that the carbon 
of that gas is applied to the purposes of nutrition. Consistently 
with this view it has been observed that plants do not thrive 
when kept in an atmosphere of pure oxygen; and it was found 
by Dr. Percival and IMr. Henry, that the presence of a little 
carbonic acid is even favourable to their growth, Saussure, 
who examined this subject minutely, ascertained that plants 
grow better in an atmosphere which contains about one-twelfth 
of carbonic acid than in common air, provided they are expo- 
sed to sunshine. But if that gas be present in a greater propor- 
tion, its influence is prejudicial: in an atmosphere consisting of 
one-half of its volume of carbonic acid, the plants perished in 
seven days; and they did not vegetate at all when that gas was 
in the proportion of two-thirds. In the shade the presence 
of carbonic acid is always detrimental. He likewise observed 
that the presence of oxygen is necessary, in order that a plant 
should derive benefit from admixture w ith carbonic acid. Saus- 
sure is of opinion that plants derive a large quantity of their car- 
bon from the carbonic acid of the atmosphere, an opinion which 
receives great weight from the two following comparative 
